So called “flat no lead packages” are becoming increasingly popular in microwave products due to, for instance, their small size, low inductance leads, and their advantageous heat sink properties. Examples of flat no lead packages are so called Quad Flat No leads packages, usually abbreviated as QFN-packages, and Dual Flat No leads packages, usually abbreviated as DFN-packages. Another term which is sometimes used for a flat no leads package is “MicroLeadFrame”.
A flat no leads package usually comprises a conducting central plate and a plurality of conducting leads which surround the central plate but are separated from it by a non-conducting material. The conducting leads form I/O ports to components outside of the flat no leads package. The flat no leads package is usually arranged on top of a circuit board, so that the conducting leads can be used as I/O-ports from the flat no leads package to the circuit board.
A drawback associated with flat no lead packages is that they exhibit so called “tie-bars”, which are needed during the manufacturing process, and which cannot be removed later, except at great cost. The tie-bars protrude from the central plate of the flat no lead package, and overlap the ground plane mentioned above, i.e. a ground plane which is usually comprised in the circuit board or arranged “below” it, as seen from the flat no lead package.
In order to understand the negative impact of the tie bars, they can be likened to a resonant stub which is short-circuited at its one end and open at its other end. Such a stub will resonate at λ/4, 3λ/4, 5λ/4, etc, where λ is the wavelength which corresponds to the frequency of the signals in the flat no lead package. Tie-bar resonances can lead to a number of problems. They can, for example, create increased leakage from one I/O-port to another, in some cases as much as 20-30 dB. Another problem is that they can lead to sharp dips in the gain of a packaged device, and to corresponding rapid variations in the group delay of the device.